ISRO Electronics-Communication-Engineering-ECE Contributed by Syam updated on Aug 2019
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ISRO electronics and electrical questions and answers with detailed explanations, ISRO technical question papers, ISRO model questions and answers, ISRO practice questions with all engineering streams

1.  In the given circuit the voltage Vc across C at time t = ? is

(A) 5

(B) 10 V

(C) 20 V    (Ans)

(D) zero

Hints and Solutions :  At state capacitor acts as a open circuit because voltage cross capacitor is equal to applied voltage i.e., VC = 20 V.

2.  Time constant of a series R-L circuit is given by

(A) LR

(B) L/R    (Ans)

(C) L2R

(D) LR2

Hints and Solutions :  For RL series circuit

i = V/R (1 - e -R/L t)

So time constant T = L/R

3.   The Q factor of a series RLC increases if

(A) R decreases     (Ans)

(B) R increase

(C) impedance increases

(D) voltage increases

Hints and Solutions :  Q = XL/R if R decreases then Q increases

4.  Which of the following is an active device ?

(A) Transformer

(B) Silicon controlled rectifier     (Ans)

(C) Electron bulb

(D) Loud-speaker.

5.  Which of the following is donor impurity element ?

(A) Aluminium

(B) Boron

(C) Phosphorus     (Ans)

(D) Indium

6.  The forbidden energy gap for silicon is

(A) 0.12 eV

(B) 1.12 eV

(C) 0.72 eV    (Ans)

(D) 0.92 eV

7.  Free electrons exist in

(A) first band

(B) second band

(C) third band

(D) conduction band      (Ans)

8.  Fig. represents a

(A) tunnel diode

(B) zener diode       (Ans)

(C) photo emissive diode

(D) photo sensitive diode

9.  The intrinsic carrier density of a Si specimen at K is

(A) 278 * 1011 cm-3      (Ans)

(B) 2.78 * 109 cm3

(C) 2.78 * 10-2 cm3

(D) 2.78 * 1019 cm3

10.  Which of the following transistor is affected by static electricity ?

(A) NPN transistor

(B) FET

(C) UJT

(D) MOSFET      (Ans)

11.  The current amplification factor (a) is given by

(A) IC / IE      (Ans)

(B) IC / IS

(C) IB / IE

(D) IB / IC

12.  Number of h parameters of a transistors are

(A) 1

(B) 2

(C) 3

(D) 4     (Ans)

13.  Number of PN junctions in JFET are

(A) 0

(B) 1

(C) 2    (Ans)

(D) 3

14.  An op-amp when used as a notch filter has a notch frequency = 2kHz, low frequency point = 2.2 kHz. The Q of the notch is

(A) 0.5

(B) 5    (Ans)

(C) 0.2

(D) 2

Hints and Solutions :  BW = 2.2 kHz - 1.8 kHz = 0.4 kHz. fr = 2 kHz

Q = fr / BW  =  2kHz / 0.4 kHz  = 5.

15.  An amplifier is used to amplify a 30 p Hz square wave with a sag not exceeding 1%. Maximum permissible lower dB frequency of the amplifier is

(A) 30 Hz     (Ans)

(B) 300 Hz

(C) 0.3  p Hz

(D) 100 Hz

16.  In which of the following applications vacuum tubes are still being used ?

(A) public address system

(C) radio transmitters     (Ans)

(D) electroplating plants

17.  Transistors have not replaced vacuum tubes because

(A) transistors are non-linear

(B) transistors do not have grid

(C) high power transistors are not available     (Ans)

(D) heat dissipation from transistors is difficult

18.  The value of L needed with C of 20 pF for an oscillator frequency of 110.7 MHz, is

(A) 0.32 ?H    (Ans)

(B) 0.5 ?H

(C) 1 ?H

(D) 5 ?H

Hints and Solutions :   ? fr = 1/2p ?LC

So,  L = 1/4pC  fr2

= 1012/4 * 3.14 * 20 * (110.7)2 * 1012 = 0.32  ?H

19.   A Wein bridge oscillator has R1 = R2 = 220 k? and C1 = C2 = 250 pF.  The frequency of oscillations will be nearly

(A) 0.89 kHz

(B) 1.89 kHz

(C) 2.89 kHz    (Ans)

(D) 3.89 kHz

Hints and Solutions :   f = 1/2p RC

= 1012/2 * 3.14 * 220 * 103 * 250 = 2.89 kHz

20.  RC network shown in Fig. is used in a low frequency oscillator circuit.  The frequency of oscillations will be

(A) 1.59 kHz

(B) 15.9 kHz   (Ans)

(C) 159 kHz

(D) 1590 kHz

Hints and Solutions :   f =  1/2p  RC   =  1/2 * 3.14 * 105 * 10-10

=   105/2 * 3.14 =   15.9 kHz.

21.  Demodulation

(A) is performed at the transmitting station

(B) removes side bands

(C) rectifies modulated signal

(D) is opposite of modulation    (Ans)

22.  A frequency multiplier stage should operate as

(A) Class A

(B) Class B

(C) Class C     (Ans)

(D) Either of the above

23.  Pulse modulation is used in

(B) automatic landing equipment

(C) data communication

(D) all of the above      (Ans)

24.  Which of the following modulating system is digital ?

(A) PPM

(B) PWM

(C) PCM      (Ans)

(D) PEM

25.  A 360 W carrier is simultaneously modulated by two audio waves with modulation percentage of 55 and 65 respectively. The total side band power radiated will be

(A) 180 watts

(B) 130 watts      (Ans)

(C) 60  watts

(D) 30 watts

26.  A radio transmitter, amplitude modulated radiates 50 kW of carrier power.  The radiated power at 85% modulation will be

(A) 50.85 kW

(B) 51.7 kW

(C) 59.9 kW

(D) 68.1 kW       (Ans)

27.  For a FM wave carrier modulating frequency is 10 kHz and band-width is 2 MHz.  If the modulating signal amplitude is doubled, the band width will be

(A) 0.5 MHz

(B) 1 MHz

(C) 2 MHz

(D) 4 MHz      (Ans)

28.  The balanced modulator circuit uses -

(A) two identical diodes

(B) two identical triodes     (Ans)

(C) two diodes with different V-I characteristics

(D) two triodes with different V-I characteristics

29.  To observe a portion of a wave form we must use

(A) free-running oscilloscope

(B) triggered-sweep oscilloscope     (Ans)

(C) vector scope

(D) cathode ray tube

30.  An oscilloscope is generally used to measure the value of

(A) dc voltage

(B) rms, ac voltage

(C) peak to peak ac voltage       (Ans)

(D) average value of ac voltage

31.  The purpose of a syne control in an oscilloscope is to

(A) set the intensity level

(B) control brightness

(C) set the focus

(D) lock the signal       (Ans)

32.  The length of the sweep on the CRT screen controlled by

(A) syne control

(B) sweep selector

(C) horizontal gain      (Ans)

(D) vertical gain

33.  Ionoscope is based on

(A) photo emission       (Ans)

(B) secondary emission

(C) photo voltic effect

(D) photo conducting principle

34.  A capacitor of capacitance c1 and distance between the plate is d1. A second capacitor of capacitance c2 and distance between the plate is d2.
When they are connected to series what is the equivalent capacitance ?

(A) d1d2 / d1 + d2

(B) d1 + d2  / d1d2

(C) c1 + c2

(D) c1 * c2  / c1 + c2       (Ans)

Hints and Solutions :  Ceq =  C1 C2  /  C1 + C2

35.  In a circuit voltage in the inductor is 2 V and inductance of the inductor is 1 mH.  What is the rate of the current decay ?

(A) 2A/S

(B) 1 * 103  A/S

(C) 2 * 103  A/S       (Ans)

(D) none of these

Hints and Solutions :  VL = L di/dt

36.  In a circuit current passing the capacitor is 1A. The capacitance of the capacitor is 1 ?F.  Find the voltage decay of the capacitor ?

(A) 1 V/S

(B) 1 * 106 V/S        (Ans)

(C) 2 * 103 V/S

(D) none of these

Hints and Solutions :  i = C  dvc / dt

dvc / dt =  i / C  =  1/1 * 10-6 =  106 Volt / sec.

37.  Two capacitors are connected in series. The first capacitor is of capacitance 40 ?F and breakdown voltage 60 V and capacitance of second capacitor is 60  ?F and break down voltage is 40 V.  What condition is satisfied ?

(A) First capacitor break down is first

(B) Second capacitor break down is first

(C) The charge equal to the both capacitor         (Ans)

(D)  None of these

38.  What is3the state equation for the nth order of differential equation,

(A) n

(B) (n + 1)        (Ans)

(C) (n + 1) / 2

(D) (n - 1)2

39.  Precision measurement of resistance is generally carried out by

(A) Potentiometer method

(B) CRO method

(C) Voltmeter-ammeter method

(D) Bridge method        (Ans)

40.  The average power flow per unit area in a uniform plane wave in an electric field of maximum voltage E0 and impedance Z0 is

(A) E02 / 2Z0      (Ans)

(B) E02 / Z0

(C) E02 / Z02

(D) I0 E0